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Lü said that his DIY trees belong in ordinary people's homes, because they can work as an air humidifier and air purifier. "The device can be popularized because all the apparatus can be manufactured according to my engineering drawings," he explained.

The couple has started regularly recording the PM 2.5 air pollution figures in the outdoor environment near his home, in his apartment and in his neighbor's apartment, which has no trees. They want to prove that the plants can help reduce PM2.5 levels and the associated health risks.

"People spend most their time in their homes, and that is why tackling the indoor PM 2.5 problem is so important," he said.

According to their data, the PM 2.5 figures in his living room are much lower than in his neighbor's room or outside. For example, on August 27 when Metropolitan visited, the outdoor PM 2.5 figure was 105 ug/L, while his home was 27 ug/L at 8 pm.

He is satisfied with the result, because "the data is lower than the level required by World Health Organization, which is the 35 ug/L."

In April 2012, environment protection volunteers from Green Beagle, a Beijing-based NGO, came to his home, and made a comparison of the PM 2.5 data in and outside. Their monitor also suggests that Lü's home has fewer particles in the air.

Feng Yongfeng, an environmentalist and founder of the Green Beagle, said residents' personal hobbies nurturing plants should be encouraged. In his opinion, however, the more fundamental measures for tackling the city's PM 2.5 problem depend on the government's efforts to reduce pollution.